Tritium ( 3 H) is essential to the construction of boosted-fission nuclear weapons. A boosted weapon contains a mixture of deuterium and tritium, the gases being heated and compressed by the detonation of a plutonium or uranium device. The D-T mixture is heated to a temperature and pressure such that thermonuclear fusion occurs. This process releases a flood of 14 MeV neutrons which cause additional fissions in the device, greatly increasing its efficiency. The tritium beta decay to 3 He (mean beta particle energy 5.7 keV; decay energy 18.6 keV) can be easily detected or can cause some other compound to fluoresce. Tritium is therefore used as a radioactive tracer element in biological research in the form of tritiated water (HTO or T 2 O) and also used in capsules surrounded by a fluo-rescing compound (e.g., zinc sulfide) to provide illumination which must be independent of the electricity supply. For example, it is used in emergency exit signs, self-luminous airport runway and helicopter pad lights, and light wands for use in directing traffic.
The low energy of the beta decay means that tritium is not an external radiation hazard because the charged decay products are stopped by 0.2 mil of water or a similar shield. However, tritium can pose an internal radiation hazard if tritiated water vapor is inhaled or absorbed through the skin. Because of its higher mass and consequent lower chemical activity, tritium gas is less strongly absorbed by the body, whether through the lungs or the skin.Nuclear physics experiments in which tritium is compared to 3He have been important to our understanding of fundamental properties of the nuclear force. Tritium is rare in nature because of its 12.4-year half-life. It is produced by cosmic radiation in the upper atmosphere where it combines with oxygen to form water. It then falls to earth as rain, but the concentration is too low to be useful in a nuclear weapons program. Most tritium is produced by bombarding 6Li [ 6 Li(n, a) 3 H] with neutrons in a reactor; it is also produced as a byproduct of the operation of a heavy-water-moderated reactor when neutrons are captured on the deuterons present. It has been suggested that it may be feasible to produce tritium in an accelerator (electronuclear breeder) in which protons bombard an appropriate target. Tritium can be stored and shipped as a gas, a metal hydride (e.g., of titanium) or tritide, and trapped in zeolites (hydrated aluminum silicate compounds with uniform size pores in their crystalline structure). Stainless-steel cylinders with capacities up to 5.6 ' 10 7 GBq (1.5 MCi) of tritium gas are used for transportation and storage and must be constructed to withstand the additional pressure which will build up as tritium gradually decays to 3 He. All five declared nuclear weapon states must have the underlying capability to manufacture and handle tritium, although the United States has shut down its production reactors due to safety considerations. Canada manufactures tritium as a byproduct of the operation of CANDU reactors. In principle, limited amounts of tritium could be made in any research reactor with the ability to accept a target to be irradiated.